Factor out common code from sweep and linear.

Factor out common code from sweep and linear.

And, some file cleanup.

Change-Id: I804db924bce3b5834f6cda481315dd2da38df5ca
Reviewed-on: https://skia-review.googlesource.com/15226
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>